The invention relates to a method and a device for determining the respective amount of fuel in the fuel tank of a motor vehicle, wherein the fuel tank has a main storage reservoir provided with a filling level meter for determining the respective fuel contained therein, and a reserve reservoir (i.e., reserve tank) which is of substantially smaller volume in comparison with the main storage reservoir, which is closed in itself, and which is supplied with fuel by a pump from the main storage reservoir.
German patent DE 32 24 919 discloses a measuring device for operating fluid storage arrangements of motor vehicles which are provided with a fuel system having a main storage reservoir and a secondary storage reservoir which is of substantially smaller volume in comparison with the main storage reservoir, which is closed in itself, and which is supplied with fuel by a pump from the main storage reservoir.
In that case, associated with the main storage reservoir and the secondary storage reservoir are a main sensor device and an auxiliary sensor device, respectively, for detecting the fuel supply. Up to the moment in time at which the main storage reservoir is completely emptied, the main sensor device, which is associated with the main storage reservoir, emits a signal specifying the filling level in the fuel tank so that, on the basis of the tank geometry, it is possible to calculate a value in respect of fuel amount, that gives a display specifying the respective amount of fuel which is present in total in the main storage reservoir and the second storage reservoir. After the main storage reservoir is completely emptied, the display of the fuel which is still present in the secondary storage reservoir is implemented by the auxiliary sensor device associated with the secondary storage reservoir. This is intended to provide a more accurate display in respect of the fuel reserve which is still present.
German published patent application DE 196 10 154 A discloses a tank display and a method for ascertaining the residual volume in a tank, which has a reservoir, referred to there as a surge pot, and an additional tank from which the fuel can be emptied into the main tank when a given low level of fuel in the main tank is reached. The respective residual volume of fuel which comes from the additional tank is calculated on the basis of fuel consumption values. This procedure may admittedly permit sufficiently accurate determination of the fuel reserve which is still present in the tank in each case. However, this is only achieved at the cost of considerable additional expenditure, as the main tank must be provided with an additional tank, which is connected to the reservoir or surge pot by way of a conduit, which can be closed by means of a valve.
In addition, German published patent application DE 38 25 630 A discloses a method for determining and displaying the storage amount of fuel contained in a storage container, in which there is provided a static sender which supplies the initial value for the display. The consumption values which mount up during operation of a motor vehicle are calculated by consumption measurement operations or by evaluation of consumptiondetermining engine characteristic values, and subtracted from that initial value. The resulting difference is displayed. In this case, the degree of accuracy of the display of the reserve which is still present is dependent, even in the reserve range, that is to say when there is only little fuel still contained in the tank, on the degree of accuracy with which firstly the entire initial volume is determined by the sender, which includes a measurement float that is subjected without a damping effect to the fluctuations in level of the storage amount, wherein the initial value of the storage amount can also be replaced by a mean value formed from sender output signals, which are detected at relatively large time intervals. DE 38 25 630 A1 does not disclose any details in regard to the configuration of the tank and the definition of a given amount of fuel which, when the tank is substantially emptied, still forms a reserve amount for a given distance to be travelled.
The invention is based on a method and a device for determining the respective amount of fuel contained in the fuel tank of a motor vehicle, as described at the outset, particularly with reference to the state of the art disclosed in DE 32 24 919 A.
An object of the present invention is to permit, by simple means and with the maximum possible level of accuracy, determination of the respective reserve which is still present in the fuel system in the last phase of emptying thereof, when fuel is only still present in the surge pot, which is also referred to as the reserve reservoir or reserve tank, in order thus to provide the driver with more accurate information about the respective amount of fuel which is still present. In this respect, the invention seeks to provide that it is not necessary for the tank used for this purpose to be fundamentally different in structure from that of conventional tanks.
This object is achieved by a method in which the residual volume of fuel disposed in the fuel tank at any given time is calculated on the basis of the fuel consumption wherein, after emptying of the main storage reservoir, the respective fuel reserve which is then still contained in the reserve reservoir is calculated as a difference from a defined volume of the reserve reservoir and the fuel consumption summed over time. The signal for establishing the moment in time from which determination of the fuel reserve in the reserve reservoir by means of consumption calculation begins is triggered in dependence on the fuel falling below a given filling level in the reserve reservoir. This object is further achieved by a device in which there is provided a first sensor which is associated with the reserve reservoir and which, after emptying of the main storage reservoir, outputs a signal triggering a calculation procedure by which the respective fuel reserve still contained in the reserve reservoir is determined as a difference from a defined volume of the reserve reservoir and the fuel consumption summed over time.
A method that is particularly desirable is one in which the moment in time at which the operation of determining the reserve in the reserve reservoir by consumption calculation begins is established in dependence on the filling level in the main storage reservoir and a given filling level in the reserve reservoir, for example in such a way that a filling level meter associated with the main storage reservoir outputs a signal which states that the main storage reservoir is empty and, in addition, a signal is triggered by a sensor associated with the reserve reservoir when the level falls below the given filling level in the reserve reservoir. The sensor is desirably arranged beneath the maximum filling level in the reserve reservoir, so that consumption calculation only begins when the level of liquid in the reservoir is at a spacing from the upper boundary thereof, so that surge movements of the liquid as a result of travel dynamics of the vehicle have a less severe effect. Furthermore, the fact that the sensor in the reserve reservoir is not associated with the maximum possible filling level therein prevents the reserve calculation procedure being initially started and then cut off again when, with the main reservoir almost empty, small amounts of fuel are still being conveyed into the reserve reservoir, for example in dependence on the travelling conditions of the vehicle, so that the filling level reaches the sensor again. The arrangement of the sensor in the reserve reservoir should therefore be so selected that, under normal operating conditions, no further fuel is conveyed out of the main storage reservoir into the reserve reservoir after the level of fuel has fallen below the filling level determined by the position of the sensor in the reserve reservoir, whereby calculation of the fuel consumption has been triggered, to ascertain the fuel reserve.
In dependence on the dimension of the reserve reservoir, in particular in the plane in which movements of the vehicle occur, it may also be desirable to provide the reserve reservoir with internal fitments which substantially reduce movements of the fuel in the reserve reservoir. If the beginning of the moment in time at which the operation of determining the respectively present reserve by calculation of the fuel consumption begins is determined in dependence on the presence of two signals - one which indicates that the main storage reservoir is substantially empty and a later second signal which displays the filling level of the reserve reservoirxe2x80x94the amount of liquid which is still present at the beginning of the consumption calculation procedure can be determined with a level of accuracy which is adequate for all practical requirements.
In this respect, it may be desirable for the tank to be so designed that filling thereof is effected by way of the reserve reservoir. For that purpose, there may be provided a filling pipe which connects the filling opening of the tank to the reserve reservoir.
If, in the operating phase in which the fuel reserve in the reserve reservoir is ascertained by consumption measurement, fuel is added, for example from a spare fuel can, in an amount which is not sufficient to fill the reserve reservoir to the filling level corresponding to the position of the sensor, correction of the procedure for ascertaining the fuel reserve would not be possible. On the contrary, the display of the fuel reserve would remain at the value calculated prior to the refilling operation. It may therefore be desirable to associate with the reserve reservoir a second sensor which is arranged at a spacing beneath the first sensor and which thus responds to a smaller volume of fuel in the reserve reservoir. As soon as the level of fuel, in operation of the motor vehicle, falls below the level of the second sensor in the reserve reservoir, the signal which is triggered thereby can be used to calculate the fuel reserve still present in the reserve reservoir, once again by way of the consumption. Naturally, in this case, the procedure starts from an initial value which is smaller than that of the volume which corresponds to the position of the first sensor. Desirably, the second sensor is so arranged that, after a usual volume (for example 5 liters) of fuel has been added from a spare can, the second sensor is below the then-existing liquid level.
In accordance with a further embodiment of the invention, it may be desirable for the engine of the motor vehicle to be switched off automatically after the amount of fuel in the reserve reservoir falls below a given minimum amount of fuel. This is intended to prevent air, which causes operational problems to the engine, from passing into the fuel supply system for the engine, in particular in the high-pressure part thereof. Before the engine is automatically shut off, the attention of the driver should be directed, by virtue of a suitable display, to the fact that the engine is soon going to be shut off, at a sufficient time interval before the engine is shut off.
In other respects, the arrangement is such that the filling level meter associated with the main storage reservoir is re-activated as soon as, with a rising level of fuel in the main storage reservoir, the filling level meter switches from the empty display to displaying a given level of liquid in the main storage reservoir. The signal can at the same time also influence the on-board computer or the like so that computation of the fuel consumption is terminated. The computer is only re-activated for that computation when, in the course of travel of the vehicle, the prerequisites already referred to above recur, more specifically, an empty display in respect of the main storage reservoir and the level of liquid in the reserve reservoir falling below the filling level which is established by the position of the first sensor.